- Hewlett-Packard Printer User Guide

Using the VPO Application Programming Interfaces
The VPO Interfaces
Chapter 4 185
Read and Write Access to the VPO Message Stream
Figure 4-7 on page 186 shows the difference between the three types of
process that access the VPO message interface. Read-write
applications are message processing programs that read messages and
then modify attributes or create new messages depending on the input
flow. Any resulting messages are then written back to the VPO message
stream. Read-only applications only read the message flow, without
writing messages back to the interface. These applications might include
programs for statistical purposes, debugging tools for support, or a
separate presentation layer. Write-only applications are external
agents which send messages to VPO in the same way as the VPO agents.
These messages should go through the Server or Agent Message Stream
Interface.
Figure 4-7 on page 186 shows the message flow within the message
manager and the different queues used for the interface. It also shows
the Message Event flow and Application Response flow within the
display manager. When an external application has opened an interface
instance and registered, the server generates a queue and sends the ID
of the queue to the application. The application can then open and read
that queue.
Figure 4-8 on page 187 shows how the display manager and the other
processes communicate.